Electrophotographic imaging process employs a charge-retentive, photosensitive member to form an electrostatic latent image. The latent image is rendered visible by depositing toner particles thereon. The developed particles are then transferred from the latent image to a transfer material such as paper. The resultant powder image deposited on the transfer material is permanently affixed thereto by applying heat and/or pressure, or with solvent vapor.
Color electrophotographic development is achieved by sequentially repeating the development process described above for each color and superimposing the developed images onto one another.
Various developing methods for visualizing electrostatic images are known in the art. One method is known as “non-contact” or “gap-jumping” development, wherein a thin layer of toner particles adhering to a toner-carrying member separated by a “gap” from a latent image-bearing member is brought into the developing region between the toner-carrying member and the latent image-bearing member. A high voltage associated with the latent image on the latent image-bearing member exerts electrostatic forces that direct the toner particles towards the latent image. The electrostatic forces are often of insufficient magnitude to overcome the adhesion forces holding the toner particles in the thin layer on the toner-carrying member. It has been proposed to apply a high AC bias voltage to the developing region in order to overcome the adhesion forces. The AC voltage is of sufficient magnitude to peel the toner particles from the toner-carrying member and allow the toner particles to “jump” the gap between the toner-carrying member and the latent image-bearing member. The toner particles land on the latent image-bearing member to form a developed image. The reciprocal nature of an AC voltage in turn frees the toner particles adhering to the latent image-bearing member from the latent image-bearing member and exerts electrostatic forces that direct the toner particles back to the toner-carrying member. This process is repeated until the latent image area moves far away from the developing region and the toner particles settle on the latent image area. The use of an AC bias has adversary effects on print speed because the rate at which the toner particles oscillate in the developing region (i.e., AC frequency) must be significantly greater than the rate at which the latent image on the latent image-bearing member moves (i.e., the surface moving speed of the latent image-bearing member) and an increase in AC frequency often results in undesirable artifacts such as poor reproducibility of thin character and line images. Furthermore, color developing in the presence of an AC bias requires one latent image-bearing member for each color and an accumulator or some other intermediate transfer member that stores each developed “tone” image, increasing the complexity and cost of the electrophotographic imaging system.
It has been proposed to eliminate the need for AC bias and/or an accumulator using various methods (e.g., by effectively reducing the impact of toner adhesion forces on the development process and thereby facilitating toner jump, or by increasing the thickness of a latent image-bearing member and thereby increasing the potential difference between latent image areas and non-image areas).
It has also been proposed to employ multiple developing rollers to enable high speed, high quality development. However, developing rollers were either sequentially activated, or disposed to repetitively perform substantially the same functions of a single developing roller.
In order to enable high speed development and eliminate the need for an accumulator while fully taking advantage of toner adhesion forces without employing rollers that perform substantially identical functions, a novel developing method capable of further improving the performance and functionality of a single developing roller by employing ancillary developing members is expected.